Stem cells are found in all multicellular organisms and have the potential to develop into a multitude of cell types during early life and growth. Stem cells are characterized by their ability for self-renewal (i.e., maintaining their undifferentiated state during several rounds of cell division), and their potency (i.e., the ability to differentiate into specialized cell types). An adult stem cell is defined as an undifferentiated cell, found among differentiated cells in a tissue or organ that can renew itself and can differentiate to yield some or all of the major specialized cell types of the tissue or organ. The primary roles of adult stem cells in a living organism are to maintain and repair the tissue in which they are found. Scientists also use the term somatic stem cell instead of adult stem cell, where somatic refers to cells of the body (not the germ cells, sperm or eggs). Embryonic stem cells are defined by their origin (i.e., cells from the preimplantation-stage embryo).
Cell potency is a general term which describes the stem cell's ability to differentiate into different cell types. The more cell types a stem cell can differentiate into, the greater its potency. Totipotency is the ability of a single cell to divide and produce all of the differentiated cells in an organism, for example spores and zygotes. Totipotent cells are those with the greatest differentiation potential. Pluripotency refers to a stem cell that has the potential to differentiate into cells representative of any of the three germ layers: endoderm, mesoderm, or ectoderm (epidermal tissues and nervous system). Multipotency describes progenitor cells which have the gene activation potential to differentiate into multiple, but limited, cell types. Oligopotency is the ability of progenitor cells to differentiate into a few cell types. Finally, a unipotent cell is a stem cell has the capacity to differentiate into only one cell type.
In the realm of allogeneic stem cell therapies for regenerative applications, one of the most recognized and studied form of cells are mesenchymal stem cells (MSCs). These cells are generally multipotent and can be isolated from a variety of tissues, with the most common sources being adipose and bone marrow. MSCs, known for their immunosuppressive properties and ability to regenerate a variety of tissues, have been utilized for the treatment of pathologies such as graft versus host disease and in orthopedic tissue regenerative applications with relative success (Bobis, Jarocha et al. 2006; Bernardo and Fibbe 2012). However, as is the case with all adult derived stem cells, the efficacy of these cells depends on both the health and age of the donor from which the cells are isolated. In general, as people age, both the density and regenerative potential of their endogenous stem cell population declines (Caplan 1994; Zhou, Greenberger et al. 2008). Since MSCs utilized in allogeneic applications are generally harvested from cadaveric donors ranging anywhere from 20 to 85 years of age, variability in both MSC quality and efficacy is an unavoidable reality (Zaim, Karaman et al. 2012).
This is of even greater concern when cells are harvested for the purpose of allogeneic stem cell implantation, where the stem cell recipient's innate immunological responses can reduce the potential effectiveness of an already less than ideal population of stem cells (Liu, Wang et al. 2011). Furthermore, adult derived MSCs are limited in terms of their capability to differentiate into a wide range of cell types as they are typically limited to differentiation into cells of mesodermal lineage (Bobis, Jarocha et al. 2006).
Induced pluripotent stem cells (iPSCs) and embryonic stem cells are well-known stem cell sources that are commonly presented as a solution to rectify these problems. However, ethical and safety concerns, along with associated regulatory barriers, currently prevent both of these stem cell types from being recognized as an immediately available alternative to adult stem cell therapies (Okano, Nakamura et al. 2013).
In the search for improved cell sources for tissue regenerative therapies, amniotic fluid has drawn increasing interest, and studies involving cells derived from the amniotic fluid have grown in prevalence within the scientific literature. These cells are seen as an immediately available and attractive alternative to other commonly studied stem cells due to their enhanced differentiation potential relative to adult stem cells, enhanced immunosuppressive capabilities, safety relative to iPSCs, easy accessibility, and lack of ethical such as those associated with embryonic stem cells (Marcus and Woodbury 2008; Roelen, van der Mast et al. 2009; Murphy and Atala 2013). In regard to their safety and efficacy, multiple studies have found that these cells have been demonstrated to differentiate into cell types from all three germ layers; yet, they do not induce tumor formation when implanted in-vivo (In't Anker, Scherjon et al. 2003; Delo, De Coppi et al. 2006; De Coppi, Bartsch et al. 2007).
In most cases, amniotic fluid cells are isolated and characterized following amniocentesis within the second trimester. Unfortunately, amniocentesis is not a wholly viable means by which to obtain the amount of cells necessary for large-scale allogeneic cell therapies. This is due to the fact that the volume of fluid collected during amniocentesis is small, necessitating ex-vivo clonal expansion of cells and resulting in a greatly increased timetable for clinical adoption due to current regulatory barriers. In addition, the procedure itself carries various risks to both the mother and the fetus such as the induction of spontaneous abortion (Himes 1999), and its use in clinical practice may be reduced in favor of tests which allow extraction of fetal genetic material from the circulating maternal blood. In contrast, third trimester amniotic fluid can be collected in volumes of up to 400 ml and obtained during routine cesarean section without risk to either the mother or fetus. In this regard, the collection of amniotic fluid during elective cesarean section following full-term pregnancy is a more attractive alternative.
However, term amniotic fluid cells have received significantly less scientific study than cells derived from the fluid earlier in pregnancy. Furthermore, while it is known that the term amniotic fluid cell population is heterogeneous, little is known about the behaviour of particular subpopulations or appropriate methods of isolating and using such subpopulations.
What is needed in the art, therefore, are stem cells isolated from full-term amniotic fluid or prenatal membranes that represent a potentially improved population of cells for allogeneic therapies as compared to adult derived MSCs, and methods for isolating such cells.